Autoignition main gas generant

ABSTRACT

A gas generating composition of the present invention contains at least one fuel selected from amides, imides, and metal amine-based fuels, ammonium nitrate or phase stabilized ammonium nitrate, and at least one metal oxide. A gas generating system  200  containing a-gas generant in accordance with the present invention is also contemplated.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. ProvisionalApplication No. 60/761,017 having a filing date of Jan. 19, 2006.

TECHNICAL FIELD

The present invention relates generally to gas generating systems, andto gas generant compositions employed in gas generator devices forautomotive restraint systems, for example.

BACKGROUND OF THE INVENTION

The present invention relates to nontoxic gas generating compositionsthat upon combustion rapidly generate gases that are useful forinflating occupant safety restraints in motor vehicles and specifically,the invention relates to thermally stable nonazide gas generants havingnot only acceptable burn rates, but that also, upon combustion, exhibita relatively high gas volume to solid particulate ratio at acceptableflame temperatures.

The evolution from azide-based gas generants to nonazide gas generantsis well-documented in the prior art. The advantages of nonazide gasgenerant compositions in comparison with azide gas generants have beenextensively described in the patent literature, for example, U.S. Pat.Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and5,035,757, the discussions of which are hereby incorporated byreference.

In addition to a fuel constituent, pyrotechnic nonazide gas generantscontain ingredients such as oxidizers to provide the required oxygen forrapid combustion and reduce the quantity of toxic gases generated, acatalyst to promote the conversion of toxic oxides of carbon andnitrogen to innocuous gases, and a slag forming constituent to cause thesolid and liquid products formed during and immediately after combustionto agglomerate into filterable clinker-like particulates. Other optionaladditives, such as burning rate enhancers or ballistic modifiers andignition aids, are used to control the ignitability and combustionproperties of the gas generant.

One of the disadvantages of known nonazide gas generant compositions isthe amount and physical nature of the solid residues formed duringcombustion. When employed in a vehicle occupant protection system, thesolids produced as a result of combustion must be filtered and otherwisekept away from contact with the occupants of the vehicle. It istherefore highly desirable to develop compositions that produce aminimum of solid particulates while still providing adequate quantitiesof a nontoxic gas to inflate the safety device at a high rate.

The use of phase stabilized ammonium nitrate as an oxidizer, forexample, is desirable because it generates abundant nontoxic gases andminimal solids upon combustion. To be useful, however, gas generants forautomotive applications must be thermally stable when aged for 400 hoursor more at 107 degree C. The compositions must also retain structuralintegrity when cycled between −40 degree C. and 107 degree C. Further,gas generant compositions incorporating phase stabilized or pureammonium nitrate sometimes exhibit poor thermal stability, and produceunacceptably high levels of toxic gases, CO and NO.sub.x for example,depending on the composition of the associated additives such asplasticizers and binders.

Yet another problem that must be addressed is that the U.S.

Department of Transportation (DOT) regulations require “cap testing” forgas generants. Because of the sensitivity to detonation of fuels oftenused in conjunction with ammonium nitrate, many propellantsincorporating ammonium nitrate do not pass the cap test unless shapedinto large disks, which in turn reduces design flexibility of theinflator.

Yet another concern includes slower cold start ignitions of typicalsmokeless gas generant compositions, that is gas generant compositionsthat when combusted result in at least 80 weight % of gaseous combustionproducts as compared to the overall weight of the combustion products.

Many compositions containing phase stabilized ammonium nitrate containan azole-based fuel such as a tetrazole. Although proven to besatisfactory in many applications, one concern is that azole-based fuelssometimes have a relatively shorter burnout time thereby complicatingthe inflation profile requirements. Furthermore, it is also an ongoingeffort to economize the design of an inflator by increasing thefunctionality of a given composition, as an autoignition (less than 160Celsius, perhaps) and primary gas generant for example.

Accordingly, ongoing efforts in the design of automotive gas generatingsystems, for example, include other initiatives that desirably producemore gas and less solids without the drawbacks mentioned above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary inflator incorporating a composition of thepresent invention.

FIG. 2 is an exemplary gas generating system, in this case a vehicleoccupant protection system, incorporating the inflator of FIG. 1.

SUMMARY OF THE INVENTION

The above-referenced concerns are resolved by gas generating systemsincluding a gas generant composition containing phase stabilizedammonium nitrate, stabilized in a known manner, metal oxides includingtransitional metal oxides such as copper oxide, and a non-azole fuel,that is a fuel not containing tetrazole, triazoles, furazans, or azoles.Accordingly, typical fuels include amides and imides such asazodicarbonamide for example, or metal amine-based fuels such as copperdiamine dinitrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The above-referenced concerns are resolved by gas generating systemsincluding a gas generant composition containing phase stabilizedammonium nitrate, stabilized in a known manner, metal oxides includingtransitional metal oxides such as copper oxide, and a fuel havingnon-azole character, that is a fuel not containing, or a fuel absent ofany tetrazoles, triazoles, furazans, or azoles. Stated another way, thegas generant composition may be described as having a non-azolecharacter because it does not contain an azole-based fuel as describedherein. Accordingly, typical fuels include at least one of amides andimides such as dihydrazides, hydrazides, succinic dihydrazide,hydrazodicarbonamide, dicyandiamide, urea, carbohydrazide, oxamide,oxamic hydrazide, Bi-(carbonamide)amine, azodicarbonamide, derivativesthereof, d- or l-tartaric acid amide derivatives, and mixtures thereof,for example; metal amine-based fuels such as copper diamine di-nitrate;and mixtures thereof. Exemplary methods of stabilizing the phasestabilized ammonium nitrate include co-crystallization of the ammoniumnitrate with potassium salts (e.g. KNO3 at about 10-15% by weight of thetotal weight of the PSAN), or by the solid-state melting of ammoniumnitrate with transition metal oxides.

Ammonium nitrate or phase stabilized ammonium nitrate (PSAN) is providedat about 60-80%, and more preferably at about 65-75% by weight of thetotal composition. A metal oxide is provided at about 2-10%, and morepreferably at about 3-7%, by weight of the total composition. The fuelis provided at about 18-38% by weight of the total composition. It willbe appreciated that the various percentages may be varied based ondesign requirements such as autoignition temperature and burn rate.

One embodiment includes 68.27% PSAN, 3.25% copper oxide, and 27.50%azodicarbonamide. The resulting gas generation is 94.9% of the totalcombustion products. It will further be appreciated that a typical dryblend ratio of PSAN to the metal oxide is about 10 to 1 respectively,but may be modified as per the weight percents described above.Differential Scanning Calorimeter (DSC) laboratory results indicate acomposition containing ammonium nitrate melt phase stabilized withcopper oxide, combined with azodicarbonamide, exhibits an autoignitiononset temperature of 150.20C, with a peak autoignition temperature of155.48C. In contrast, nitrocellulose (smokeless powder) indicates anonset of 189.35C with a peak temperature of 214.19C. Accordingly,auto-ignition occurs relatively lower with compositions of the presentinvention.

In yet another aspect of the invention, the present compositions may beemployed within a gas generating system. For example, a vehicle occupantprotection system made in a known way contains crash sensors inelectrical or operable communication with an airbag inflator in asteering wheel or otherwise within the vehicle, and also within aseatbelt assembly. The gas generating compositions of the presentinvention may be employed in both subassemblies within the broadervehicle occupant protection system or gas generating system. Morespecifically, each gas generator employed in the automotive gasgenerating system may contain a gas generating composition as describedherein.

It should be noted that all percents given herein are weight percentsbased on the total weight of the gas generant composition. The chemicalsdescribed herein may be supplied by companies such as Aldrich ChemicalCompany and Polysciences, Inc. for example.

As shown in FIG. 1, an exemplary inflator incorporates a dual chamberdesign to tailor the force of deployment an associated airbag. Ingeneral, an inflator, containing a primary autoigniting gas generatingcomposition 12 formed as described herein, may be manufactured as knownin the art. U.S. Pat. Nos. 6,422,601, 6,805,377, 6,659,500, 6,749,219,and 6,752,421 exemplify typical airbag inflator designs and are eachincorporated herein by reference in their entirety. It should also beappreciated that with regard to thermal stability and USCARrequirements, it has been found that the use of desiccant, such aszeolite, provided at about 1:1 weight ratios with regard to the gasgenerant 12, improves the thermal stability of the present compositions.Co-owned and co-pending U.S. application Ser. No. 11/604,628 filed onNov. 27, 2006, incorporated herein by reference, further explains howthe use of a desiccant may provide thermal stability advantage.

Referring now to FIG. 2, the exemplary inflator 10 described above mayalso be incorporated into a gas generating system such as an airbag orvehicle occupant protection system 200. Airbag system 200 includes atleast one airbag 202 and an inflator 10 containing a gas generantcomposition 12 in accordance with the present invention, coupled toairbag 202 so as to enable fluid communication with an interior of theairbag. Airbag system 200 may also include (or be in communication with)a crash event sensor 210. Crash event sensor 210 includes a known crashsensor algorithm that signals actuation of airbag system 200 via, forexample, activation of airbag inflator 10 in the event of a collision.

Referring again to FIG. 2, airbag system 200 may also be incorporatedinto a broader, more comprehensive vehicle occupant restraint system 180including additional elements such as a safety belt assembly 150.

FIG. 2 shows a schematic diagram of one exemplary embodiment of such arestraint system. Safety belt assembly 150 includes a safety belthousing 152 and a safety belt 100 extending from housing 152. A safetybelt retractor mechanism 154 (for example, a spring-loaded mechanism)may be coupled to an end portion of the belt. In addition, a safety beltpretensioner 156 containing propellant 12 and autoignition 14 may becoupled to belt retractor mechanism 154 to actuate the retractormechanism in the event of a collision. Typical seat belt retractormechanisms which may be used in conjunction with the safety beltembodiments of the present invention are described in U.S. Pat. Nos.5,743,480, 5,553,803, 5,667,161, 5,451,008, 4,558,832 and 4,597,546,incorporated herein by reference. Illustrative examples of typicalpretensioners with which the safety belt embodiments of the presentinvention may be combined are described in U.S. Pat. Nos. 6,505,790 and6,419,177, incorporated herein by reference.

Safety belt assembly 150 may also include (or be in operablecommunication with) a crash event sensor 158 (for example, an inertiasensor or an accelerometer) including a known crash sensor algorithmthat signals actuation of belt pretensioner 156 via, for example,activation of a pyrotechnic igniter (not shown) incorporated into thepretensioner. U.S. Pat. Nos. 6,505,790 and 6,419,177, previouslyincorporated herein by reference, provide illustrative examples ofpretensioners actuated in such a manner.

It should be appreciated that safety belt assembly. 150, airbag system200, and more broadly, vehicle occupant protection system 180 exemplifybut do not limit gas generating systems contemplated in accordance withthe present invention.

The compositions may be dry or wet mixed using methods known in the art.The various constituents are generally provided in particulate form andmixed to form a uniform mixture with the other gas generantconstituents. The mixture is then pelletized or formed into other usefulshapes in a safe manner known in the art.

In one aspect of the invention, it has been found that forming a complexbetween ammonium nitrate and the metal oxide, copper oxide for example,may best be accomplished by melting the two compounds and thenhomogeneously mixing the melt. A heating/mixing vessel may be employedwherein ammonium nitrate, or phase stabilized ammonium nitrate, isheated to its melting point. It has been found that heating ammoniumnitrate or phase stabilized ammonium nitrate (co-precipitated with 10-15wt % potassium nitrate for example) at about 150-175C provides asufficient melt. Copper oxide, or any other metal oxide such as atransitional metal oxide, is then mixed in and melted as well. Thecontents of the vessel may be stirred and heated to complex the copperor copper oxide with the ammonium nitrate or phase stabilized ammoniumnitrate. After stirring to provide a substantially homogeneous mixture,about 15-20 minutes for example, the heat is removed and the melt ispreferably slowly cooled to room temperature. After the melt solidifiesinto the copper complex, the solid may be ground by mortar and pestle,or other known grinding techniques. Powdered fuel and powdered complexmay then be homogeneously mixed in a planetary mixer for example, andthen compacted and pelletized in a known manner. The melt constituentsare of course provided in the weight percents characterized herein.

It should be noted that all percents given herein are weight percentsbased on the total weight of the gas generant composition. The chemicalsdescribed herein may be supplied by companies such as Aldrich ChemicalCompany and Polysciences, Inc., or Toyo Kasie Kogyo Co. of TakasagoCity, Japan, for example. Or, the various constituents may be made asknown in the art. For example, d- or l-tartaric acid amide derivativesmay be formed as described in U.S. Pat. No. 5,306,844, hereinincorporated by reference in its entirety.

In sum, the present invention provides simplification of the inflatordesign by only requiring a gas generating composition (auto-ignitingbelow 200C), rather than an auto-ignition composition and a separate gasgenerating composition. Furthermore, in many gas generators orinflators, a booster composition must also be employed to provide theenergy needed to combust the primary gas generant in the event of afire. By eliminating the need for an auto-igniting composition, the needfor a booster composition may also be eliminated if desired.Furthermore, decomposition products typically associated with thedecomposition of the auto-ignition composition in typical inflators isavoided. As such, the integrity of the propellant and the performancereliability of the inflator are favorably enhanced.

The present description is for illustrative purposes only, and shouldnot be construed to limit the breadth of the present invention in anyway. Thus, those skilled in the art will appreciate that variousmodifications could be made to the presently disclosed embodimentswithout departing from the scope of the present invention as defined inthe appended claims.

1. A gas generant composition comprising: a fuel selected from the groupof amides, imides, metal amine-based fuels, and mixtures thereof;ammonium nitrate; and a metal oxide, wherein said gas generantcomposition has a non-azole character.